From a movie: Cart ^^ 
The song just gave me shivers. Cuz D.O sang it *_*

Check it out V
Too many to be  listed.....
Every song from It's Okay that's Love <3
Kill Me Heal Me 
Healer
School 2015
You Who Came From The Stars
Marriage Not Dating
my favorite ost:
that winter the wind blows - the one
cruel city - everyday & hurt
heartstrings - beacause i miss you
sad love story - even when we seperate so many times
secret garden - here i am & that man
i remember you - dear cloud
Hands down The Princess' Man -- all the music was fantastic. 

Also really liked:

Fated to Love You (Korean)
Autumn's Concerto
I Hear Your Voice
Kill Me, Heal Me
Secret
The Greatest Love
Too many singles that are great! Sooo I rated personally on the whole drama soundtrack.  The list is ever changing only because I have not seen every drama out there and I will watch some mentioned above simple from the ost likes above. So most memorable soundtracks for me so far are:
Fated To Love - if you seen it. Need I say anymore
It's Okay it's Love- Just that good!
Secret Garden- How can  you not fall in love
Rooftop Prince- Just unforgettable
School 2015 
I love ALL ost in that drama 
All songs from Master Sun.
All songs from Master Sun.
Auditory Hallucination by Jang Jae In. OST of Kill Me, Heal Me <3
My favorite are from the dramas
boys over flowers 
heirs
mary stayed out all night
dream high
goong 
I Think I by Byul (ost Full House)
Perhaps Love by Howl and J (ost Princess Hours)
Spring Rain by Baek Ji Young (ost Gu Family Book)
You Don't Love Me by Roy Kim (ost Second Time Twenty Years Old)
Hello, Goodbye by Hyo Rin (ost You Who Came From The Stars)
Pinocchio - Roy Kim (Pinocchio)
Sorrow Song - Yim Jae Bum (Live in Love, Jang Ok Jung)
Secret Paradise - Jang Jae In (Scholar Who Walks Through the Night)